1. Field of the Invention
The present invention relates to a plasma-processing apparatus in which plasma discharge by means of a high frequency power with a frequency belonging to a very high frequency (VHF) band region (this frequency will be hereinafter simply referred to as “VHF”) is used, including a plasma CVD apparatus or a plasma etching apparatus, which is used for the production of a semiconductor representatively such as an amorphous silicon series semiconductor, a crystalline silicon series semiconductor or the like.
2. Related Background Art
There are known a number of plasma-processing apparatus in which plasma discharge by means of a high frequency power is used, including a plasma CVD apparatus and a plasma etching apparatus, wherein as said high frequency power, a high frequency power with a frequency of 13.56 MHz is generally used.
In general, such a plasma-processing apparatus basically has a vacuum chamber having a reaction space formed between a high frequency power application electrode and a counter electrode (an anode) and a high frequency power source, which is connected to said high frequency power application electrode through a matching box and a power supply cable, wherein a high frequency power with a frequency of 13.56 MHz from the high frequency power source is supplied to the high frequency power application electrode and simultaneously with this, a gas is introduced into the reaction space, to cause a discharge in the reaction space to produce plasma in the reaction space, whereby an object to be plasma-processed, such as a substrate, which is arranged in the reaction space, is plasma-processed by said plasma.
In the plasma-processing apparatus, which is operated in this way, there is a tendency in that a capacitance formed between the high frequency power application electrode and the counter electrode (as an earth electrode) and that formed between the high frequency power application electrode and members (including the inner wall face of the vacuum chamber) with earth potential, which are present in the peripheries of the high frequency power application electrode, are together formed at the high frequency power application electrode. Further, in the case where an insulation member or the like, which is made of ceramic, is adopted in the electrode structure, a capacitance of a magnitude that cannot be disregarded occurs at the high frequency power application electrode.
These capacitances become such that they are equivalently connected in parallel with the resistance of the plasma impedance to lower the value of the plasma impedance to afford a capacitive load. This capacitive load increases the transmitting current in the power transmission path, where Joule heat occurs in the power supply cable and the like to increase the power loss in the high frequency power supplied to the high frequency power application electrode.
In order to solve such a problem, there are several proposals. For instance, Japanese Laid-open Patent publication No. Hei 4 (1992)-237940 (hereinafter referred to as “Document 1”) discloses a manner for diminishing such Joule heat occurring in the high frequency power transmission path. Particularly, Document 1 discloses a plasma generation apparatus having a vacuum vessel and a high frequency power application electrode provided in the vacuum vessel and which is connected to a high frequency power source through a power supply capable, wherein a plasma is generated in the vacuum vessel by virtue of a high frequency power supplied to the high frequency power application electrode through the power supply cable. In this plasma generation apparatus, as the means to cancel the capacitance of the plasma impedance and the capacitance, which is formed between the high frequency power application electrode and the inner wall face of the vacuum chamber and the like that are with the earth potential, separately from the power supply cable that is connected to the high frequency power application electrode, a variable-length coaxial pipe whose tip is short-circuited by a series resonance circuit comprising LC is connected to the high frequency power application electrode as a dielectric stub for the high frequency power application electrode.
Japanese Patent Publication No. Hei 1 (1989)-19254 (hereinafter referred to as “Document 2”) discloses an electrode structure capable of diminishing the capacitance formed between the high frequency power application electrode and the members (including the inner wall face of the vacuum chamber) with earth potential, which are present in the peripheries of the high frequency power application electrode. Particularly, Document 2 discloses a plasma-processing apparatus provided with an electrode structure having a pair of plane parallel plate electrodes for the application of a high frequency power, which are arranged to oppose to each other through a plasma generation region. One of the two plane parallel plate electrodes is grounded (earthed) to have the earth potential. At least one metal plate, which is insulated from the non-earthed plate electrode and also from the conductors present in the peripheries of the non-earthed plate electrode, is inserted to diminish the capacitance formed between the non-earthed plate electrode and the conductors present in the peripheries of the non-earthed plate electrode.
Document 2 describes that the electrode structure makes it possible to diminish the inter-electrode capacitance between the high frequency power application electrode and the conductors excluding the counter electrode and prevents discharge and generation of plasma at unnecessary portions.
It should be noted that in recent years, attempts have been made in order to achieve the formation of a non-crystalline (amorphous) silicon thin film or a crystalline silicon thin film on a substrate at an improved deposition rate using the plasma-processing apparatus in which a high frequency plasma generated by means of a high frequency power with a frequency of 13.56 MHz is used.
However, because the high frequency plasma generated by means of a high frequency power with a frequency of 13.56 MHz, which belongs to a HF band region, has a relatively small energy, which is incapable of achieving a high deposition rate. In fact, the deposition rate that makes it possible to deposit a high quality non-crystalline silicon film or a high quality crystalline film on a substrate is several Angstroms (Å)/sec or less.
In order to more raise the deposition rate, it is necessary to increase the density of the plasma generated. For this purpose, it is necessary to apply a large high frequency power with a frequency belonging to a VHF (very high frequency) band region or a frequency band region greater than said VHF band region to the high frequency power application electrode of the plasma CVD apparatus.
Separately, in order to further improve the productivity of a large area semiconductor device, such as a display or a solar cell in which a semiconductor comprising a crystalline silicon thin film or a non-crystalline silicon thin film is used and which excels in the quality and performance, it is necessary to uniformly deposit a crystalline silicon thin film or a non-crystalline silicon thin film, which has excellent quality and property over a large area, at an improved deposition rate.
For achieving this purpose, it is necessary to use high frequency plasma whose energy is greater than that generated by high frequency power with a frequency of 13.56 MHz, which belongs to a HF band region.
In this connection, various studies have been made in order to achieve high speed film formation over a large area using plasma generated by a large high frequency power with a very high frequency (VHF) or a frequency belonging to a microwave band region. For this purpose, it is required that such large high frequency power is supplied uniformly to the entire region of a large high frequency power application electrode having a large area so as to generate uniform plasma having a high density over a large area.
In the case where a deposited film is intended to form uniformly over a large area substrate using a plasma generated by such large high frequency power with, for instance, VHF (this plasma will be hereinafter referred to as “VHF plasma”), it is necessary to enlarge the high frequency power application electrode so that it has a larger area than the large area substrate. In this case, when the technique described in Document 1 is adopted, such problems as will be described in the following are likely to occur. When a high frequency power with a VHF (hereinafter referred to as “VHF power”) is supplied to the high frequency power application electrode through the power supply cable, the Joule heat in the high frequency power transmission path is increased to greatly increase the power loss. In addition, because the inductance component is excessively large, the electric field distribution at the face of the high frequency power application electrode becomes uneven and plasma is hardly generated. Thus, it is impossible to generate uniform VHF plasma over a large area. In order to generate uniform VHF plasma over a large area using VHF power, it is necessary to consider that the VHF plasma has a larger energy than that generated by a high frequency power with a frequency of 13.56 MHz. In addition, the high frequency power application electrode through which a VHF power is supplied must be considered as a distributed constant circuit. Particularly, it is necessary to provide the position for the VHF power to be supplied to the high frequency power application electrode. In addition, it is necessary to provide an impedance-matching equipment capable of effectively diminishing the capacitance of the high frequency power application electrode. It is also necessary to determine how the impedance-matching equipment should be structured and how the impedance-matching equipment should be positioned with respect to the high frequency power application electrode.
Unless these factors are considered, it is difficult to stably and continuously generate uniform VHF plasma such that it is uniformly distributed over a large area at a uniform density, where it is almost impossible to deposit a high quality film over a large area substrate at a uniform thickness and at a high deposition rate.
Thus, in accordance with the technique described in Document 1, it is difficult to stably and continuously generate a uniform VHF plasma over a large area at a uniform density so that a high quality film can be deposited uniformly over a large area substrate at a high deposition rate.
Separately, in the case where the high frequency power application electrode is enlarged to have a large area in order to make it possible to supply a VHF power which is a large high frequency power, it is difficult to decrease the capacitance of the electrode to a sufficient level in accordance with the technique described in Document 2, where problems are likely to occur in that, particularly, the power introduction portion of the high frequency power application electrode is overheated and the high frequency power application electrode is damaged.
Thus, in accordance with the technique described in Document 2, it is also difficult to stably and continuously generate a uniform VHF plasma over a large area at a uniform density so that a high quality film can be deposited uniformly over a large area substrate at a high deposition rate.